The famous document signed at Runnymede in 1215 was the Magna Carta. This charter was a landmark agreement between King John of England and a group of rebel barons. It established the principle that everyone, including the king, was subject to the law and guaranteed certain legal rights, such as the right to a fair trial. The Magna Carta is often considered a foundational document in the development of modern democracy and the rule of law, influencing legal systems and constitutional frameworks in many countries around the world.